(display "\n========================================\n")
(define (add-interval x y)
    (make-interval (+ (lower-bound x) (lower-bound y))
                   (+ (upper-bound x) (upper-bound y))))

(define (sub-interval x y)
    (make-interval (- (lower-bound x) (upper-bound y))
                   (- (upper-bound x) (lower-bound y))))

(define (make-interval a b) (cons a b))
(define (lower-bound z) (car z))
(define (upper-bound z) (cdr z))

(define v1 (make-interval 1 2))
(define v2 (make-interval 3 4))
(display (sub-interval v1 v2))
(display "\n========================================\n")
